In recent years, there have been studied natural energy such as coal energy, biomass energy, nuclear energy or natural energy of wind power energy, solar energy or the like, in place of fossil fuels such as petroleum, natural gas or the like, and the natural energy which is most stable as alternative energy for fossil fuels and in a large amount is considered to be solar energy.
However, although solar energy is very important alternative energy, it supposedly becomes a problem that (1) the energy density of solar energy is low and (2) storage and transfer of solar energy are difficult.
On the other hand, it was proposed that the problem of density of solar energy being low was overcome by collecting solar energy by giant reflection devices.
There was conventionally used a glass mirror as a reflection device since it was exposed to ultraviolet rays or heat of solar light, weather, sandstorms or the like. Such glass mirrors are high in environmental durability, however they are easily broken at the time of transportation or heavy in mass, which necessitates to enhance the strength of a rack to install the mirror, producing problems such that the solar plant construction cost increases.
To overcome the foregoing problems, it was proposed to replace a glass mirror by a resin reflection sheet (as described in, for example, patent document 1). However, such a resin is weak to the external environment and when a metal such as silver is used in a reflection layer, oxygen, moisture or hydrogen sulfide transmits through the resin layer, producing problems such that silver is corroded; therefore, it was difficult to make use of a resin mirror.
In the object of collecting solar light, it is preferred to constitute a metal layer with silver exhibiting a high reflectance in the visible region in terms of achieving enhanced reflectance, as described in patent document 2. However, there is the problem that silver is inferior in weather resistance and easily deteriorated with oxygen, moisture or sulfur.
With respect to such a problem, it is anticipated in patent document 2 that a plastic substrate functions as a protective layer for the silver layer. However, plastics easily transmit water vapor or oxygen in the air, so that silver is deteriorated by oxidation, leading to problems such that reflectance of a mirror is lowered.
When using a mirror to reflect sun light, the mirror is often used outside. When using a mirror outdoors, the mirror is exposed to the weather and oxidation deterioration of silver is accelerated under such a severe environment and a lowering of reflectance of the mirror becomes a marked problem.
With respect to a technique for corrosion inhibition of silver, there has been known a technique of coating a corrosion inhibiting layer such as a resin layer as an adjacent layer on the light incident face side (as described in, for example, patent document 3). However, even in such a technique, when used as a mirror for solar light reflection, the mirror is placed under severe outdoor environments and even such a technique could not realize a sufficient effect against corrosion, leading to insufficient dissolution of lowering of reflectance.
Further, there was proposed, as a means for preventing deterioration of the silver mirror, a technique of providing a layer containing a ultraviolet absorber and a corrosion inhibitor as an upper layer for the silver layer (as described in, for example, patent document 4).
However, when a film mirror is used as a reflection device for solar power generation, the mirror is always exposed to the weather or sun light and is used under conditions in which temperature and humidity greatly vary, as compared to the use for a conventional mirror or the like. When adopting the foregoing technique under such conditions, it was proved that “floated off” or “peel-off” of a layer occurs between the layer containing the corrosion inhibitor and the adjacent layer or deformation is generated at the interface to the adjacent silver reflection layer, sometimes leading to a lowering of specular reflectance.
On the other hand, when used as a mirror for solar light reflection, there is a problem such that, as the UV absorption function of a UV absorber is weakened through secular change, deterioration of silver proceeds from the side farthest from the light incident surface, resulting in a lowering of specular reflectance. In a reflection mirror for use in solar power generation, it is required to condense solar light to an intended position, so that it is necessary to maintain not only high reflectance but also enhanced specular reflectance and it was proved that deterioration of silver from the backside caused lowering of specular reflectance, resulting in problems. Further, in the patent document 4, there was disclosed a technique to inhibit corrosion of silver in which a metal layer of copper or the like exhibiting a higher ionization tendency than silver is provided as an adjacent layer to silver of the side farther from the light incident surface to form a victim layer for anti-corrosion of silver. However, it was proved that, when such a technique was applied, there was produced a problem that, since such a victim layer is more corrosion-prone than silver, deterioration of the victim layer proceeds, resulting in deformation of the victim layer, and as a result, the specular reflectance is lowered, rendering it difficult to maintain the desired high specular reflectance of the mirror.